mainlining shenanigans
26e03d803c
On rk3399-gru-kevin there are some cases where we're seeing AUX CH failures when trying to do DisplayPort over type C. Problems are intermittent and don't reproduce all the time. Problems are often bursty and failures persist for several seconds before going away. The failure case I focused on is: * A particular type C to HDMI adapter. * One orientation (flip mode) of that adapter. * Easier to see failures when something is plugged into the _other type C port at the same time. * Problems reproduce on both type C ports (left and right side). Ironically problems also stop reproducing when I solder wires onto the AUX CH signals on a port (even if no scope is connected to the signals). In this case, problems only stop reproducing on the port with the wires connected. From the above it appears that something about the signaling on the aux channel is marginal and any slight differences can bring us over the edge to failure. It turns out that we can fix our problems by just increasing the voltage swing of the AUX CH, giving us a bunch of extra margin. In DP up to version 1.2 the voltage swing on the aux channel was specced as .29 V to 1.38 V. In DP version 1.3 the aux channel voltage was tightened to be between .29 V and .40 V, but it clarifies that it really only needs the lower voltage when operating at the highest speed (HBR3 mode). So right now we are trying to use a voltage that technically should be valid for all versions of the spec (including version 1.3 when transmitting at HBR3). That would be great to do if it worked reliably. ...but it doesn't seem to. It turns out that if you continue to read through the DP part of the rk3399 TRM and other parts of the type C PHY spec you'll find out that while the rk3399 does support DP 1.3, it doesn't support HBR3. The docs specifically say "RBR, HBR and HBR2 data rates only". Thus there is actually no requirement to support an AUX CH swing of .4 V. Even if there is no actual requirement to support the tighter voltage swing, one could possibly argue that we should support it anyway. The DP spec clarifies that the lower voltage on the AUX CH will reduce cross talk in some cases and that seems like it could be beneficial even at the lower bit rates. At the moment, though, we are seeing problems with the AUX CH and not on the other lines. Also, checking another known working and similar laptop shows that the other laptop runs the AUX channel at a higher voltage. Other notes: * Looking at measurements done on the AUX CH we weren't actually compliant with the DP 1.3 spec anyway. AUX CH peek-to-peek voltage was measured on rk3399-gru-kevin as .466 V which is > .4 V. * With this new patch the AUX channel isn't actually 1.0 V, but it has been confirmed that the signal is better and has more margin. Eye diagram passes. * If someone were truly an expert in the Type C PHY and in DisplayPort signaling they might be able to make things work and keep the voltage at < .4 V. The Type C PHY seems to have a plethora of tuning knobs that could almost certainly improve the signal integrity. Some of these things (like enabling tx_fcm_full_margin) even seem to fix my problems. However, lacking expertise I can't say whether this is a better or worse solution. Tightening signals to give cleaner waveforms can often have adverse affects, like increasing EMI or adding noise to other signals. I'd rather not tune things like this without a healthy application of expertise that I don't have. Signed-off-by: Douglas Anderson <dianders@chromium.org> Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.